Principles of Fluorescence Spectroscopy

340 MECHANISMS AND DYNAMICS OF FLUORESCENCE QUENCHING
Figure 9.15. Correlation of k q for PET quenching of Neutral Red with
∆G for the reaction. Revised from [59].
k q because k p (r) is much larger than k q and PET is limited
by the rates of diffusion. In order to study the electrontransfer
rate it is necessary to eliminate the effect of diffusion.
However, high quencher concentrations are required
Figure 9.16. Time-dependent decays of coumarin derivatives in neat
DMA. Revised and reprinted with permission from [60]. Copyright ©
1993, American Chemical Society.
to obtain contact quenching without diffusion. The intensity
decays of the three coumarin derivatives were examined
in neat DMA (Figure 9.16). 60 In pure DMA the fluorophores
are surrounded by electron donors so no diffusion
is needed to bring D P and A P into contact. Hence the intensity
decays should reflect the electron-transfer rate k p (r)
rather than the diffusion-controlled collision rate. The most
rapid intensity decay was observed for C151 where ∆G =
–15.5 kcal/mole. Slightly slower decays were observed for
C152 and C522, where ∆G is less negative.
9.3.2. PET in Linked Donor–Acceptor Pairs
The need for high quencher concentrations can be avoided
using covalently linked D–A pairs. One example is shown
in Figure 9.17, where the electron acceptor methylviologen
(MV 2+ ) is covalently linked to 1,8-naphthalimide. The spacer
contained n = 2 to 6 methylene groups. 61 In this case the
naphthalimide fluorophore is the electron donor and MV 2+
Figure 9.17. Emission spectra of covalently linked 1,8-naphthalimide
and methylviologen. n is the number of methylene groups. The control
compound lacked the methylviologen acceptor, and instead had a
linked pyridine group with the same number of methylene groups.
Revised and reprinted with permission from [61]. Copyright © 2000,
American Chemical Society.